It is known to stretch a yarn, by which is meant a filament formed of a multiplicity of fibers, by passing it between at least two gripper-roller assemblies that each include at least two gripper rollers and that operate at different speeds to stretch the yarn longitudinally in at least one stretching zone. A sensor is juxtaposed with the yarn and is connected through an appropriate controller and drive to one of the roller assemblies so that its speed can be varied to increase the stretch in the yarn when its mass increases at the measuring location and to decrease it when its mass decreases. In this manner irregularities in the size and weight of the yarn can be eliminated.
Operating speeds have increased in recent times to levels which make this system fairly ineffective, as an intolerably thick or thin portion of yarn can pass through to the spinner before corrective action can be taken. No matter how sensitive and fast-acting the sensor is, it is impossible to obtain fast enough reaction from the mechanical drive elements to produce a product of high uniformity when working at such high speed.
German patent document No. 2,912,576 based on Swiss application No. 4497-78 filed Apr. 26, 1978 by W. Grunder describes a system having sensors at several locations forming an input for a controller/drive unit that in turn can control the rotation rates of several different roller assemblies. One sensor is constructed to detect short-period variations and the other long-period variations and both produce outputs that are combined by the controller/drive to control the operation rate of a single stretch zone. The operation speed of such a system is still regrettably slow.
A carding system is seen in German patent document No. 1,921,248 based on Swiss application No. 9728-68 filed June 28, 1968 by E. Felix. This arrangement uses short-period and long-period sensors which act on different stretching zones. Similarly a fluted-roll arrangement is seen in U.S. Pat. No. 3,694,861 of J. Whitehurst wherein short-period mass variations in a sliver being stretched are detected and responded to.
None of these systems is capable of producing a product that not only is uniform, but uniform at a particular size or yarn number. Thus some can produce a relatively uniform product, but one whose gauge will vary over a long period within a fairly wide range, while others will produce a yarn whose gauge will vary rapidly and often in a relatively narrow range.